Molecular sieves, especially, zeolites, have been used in a number of commercial processes. For examples, the production of xylenes via transalkylation of C9+ aromatics with C6-C7 aromatics to produce xylenes uses a zeolite based catalyst comprising at least one of Mordenite, ZSM-12, MCM-22 family material, and any combination thereof. Typically molecular sieves lose performance, such as activity, selectivity, and capacity, through various deactivation mechanisms. As the molecular sieve catalyst or adsorbent ages with increasing time on stream, more severe conditions, such as, higher temperature and/or low through-put, are normally required to maintain comparable activity and/or selectivity. When the maximum reactor temperature is reached, the molecular sieve catalyst or adsorbent needs to be replaced or regenerated. The spent catalyst, at the end of its useful life, may contain a significant amount of coke, often exceeding 35 wt. %, and sometimes even as high as 50 wt. %. The coke deposit may be graphitic in nature, characterized by having a low H/C ratio, and is often difficult to remove during regeneration under standard regeneration conditions. One common regeneration technique is to burn the coke from the molecular sieve in an oxidative environment, such as air or oxygen. However, the oxidative calcination process normally needs to be controlled with dry air to prevent steam damage to the molecular sieve framework, for example, steam dealumination of zeolite, which causes severe damage to the molecular sieve. Another less common regeneration technique is to rejuvenate a spent catalyst in a reductive environment, such as hydrogen. However, if the catalyst is not regenerated properly, aging rates during the second cycle can be very high resulting in a second cycle length as short as less than 10% of the first cycle length.
Other regeneration techniques include the use of steam or other solutions in combination with heating or calcining. For example, U.S. Pat. No. 5,093,293 discloses the use of steam for removing coke and other contaminants from Zeolite L. U.S. Pat. No. 4,139,433 discloses that a hydrocracking catalyst containing a Group VIII metal is regenerated by treating the spent catalyst with an ammonium hydroxide solution followed by calcination at 500° F. to 950° F. It is stated that the process redistributes the Group VIII metals and removes mono and divalent metal cations.
U.S. Pat. No. 4,975,399 discloses a two-step heating process to remove carbonaceous deposits from a hydrotreating catalyst. U.S. Pat. No. 4,550,009 discloses treating a spent catalyst with a source of alkali or alkaline earth metal cations or ammonia and then extracting extractable nitrogen compounds with a liquid organic solvent.
We have surprisingly discovered that the controlled addition of water during the regeneration, combined with a staged controlled temperature burn, can successfully regenerate a heavily coked catalyst with minimal steam damage of the molecular sieve structure. In fact, the aging rate of the regenerated catalyst decreases with increased water exposure during regeneration. The benefit of water addition was unexpected and contrary to its effect during regeneration of other zeolitic catalyst systems where hydrothermal deactivation is increased with water addition. In addition, we have discovered that the rate of coke removal increases with increasing water partial pressure, thereby shortening the required regeneration time.